Hi, it seems like everything is wrong with Dead Sunlight... He used to be in a half-gallon, but I've renovated him to a 10 gallon all by himself... (Go figure) in the half- gal, he had no problems, but in the 10, he started to swim in triangles and circles, etc... And started biting his tail... Can anyone help?

That is too bad, tail biting can be a challenge, I am sure you know the most common causes are-pain from ammonia burns, parasites, injuries as well as boredom, stress to name some reasons.
I would start by making water changes to make sure its not the environment, you may even want to move him back to his other tank as the bigger tank may be stressing him out, you never know....good luck, keep the water extra clean to prevent secondary infection in the open wounds.

I think since the trigger was the change in environment it is quite unlikely to be boredom. Usually bettas have certain recurring triggers, and if you can figure it out you can reduce the tail biting behavior.

I have had three tail biters that I've controlled, one did not like complete darkness--he had to sleep with a night light to keep from biting at night. One had too much decoration in his tank and was fine after a cave was removed. Another is just a total brat who bites when I skip a feeding. Tail biting triggers are highly variable, possibly due to the irresponsible way fish are bred en masse these days.

Since this fish also has other health problems, I would definitely look into the water quality and temperature issues. The ich itself may be a bite motivator as well.